color question

We just recently had some chicks hatch out in our coop. Our flock consists of buff orps. buff cochins, and 2 americanas. The only rooster with them is a buff orpington. 2 of the hatchlings are black, with a yellow spot on the top of their head, and no feathered feat. How can this be? I do not think one of the americanas laid the egg, because they hatched out of brown eggs. These are hatchery hens, so I guess some of the hens could be mixed. Any ideas?